SEGREGATION: Causes and Cures - INTI

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INTRODUCTION
Hot mix asphalt mixtures that are properly designed, produced and placed
provide a durable, long lasting pavement that requires very little maintenance.
However, there are a number of potentially damaging problems that can occur
in the design, production and placement of hot mix paving mixtures. Of these
problems, perhaps the most serious is segregation. Segregation is a frequently
recurring problem that has caused concern within the paving industry for decades
and receives widespread attention by contractors, state highway departments
and equipment manufacturers.
When segregation is present in a mixture, there is a concentration of coarse
materials in some areas of the paved mat, while other areas contain a
concentration of fi ner materials. Segregation creates non-uniform mixes that do
not conform to the original job mix formula in gradation or asphalt content. The
resulting pavement exhibits poor structural and textural characteristics and has
a shorter life expectancy.
Problems associated with segregation are serious. Their elimination is essential
to the production of high quality paving mixtures. Elimination of segregation is
the responsibility of those who produce and lay asphalt mix, those state highway
departments who design the mix and inspect the fi nal product, and those
manufacturers who design and market machinery for the paving industry.
This paper was written to help designers, plant operators and paving crews be
aware of the causes of segregation and known solutions. Each portion of the
plant, paver and trucking operation known to cause segregation is discussed
separately. In addition, a diagnostic chart accompanies this paper to help identify
types of segregation and probable causes.
MIX DESIGNS
Proper mix design is important in the effort to eliminate segregation. Mixes that
are uniformly designed with no gap-grading are generally very forgiving. They
allow mistakes in other areas of the plant operation or laydown operation without
affecting the mix performance signifi cantly.
Gap-graded mixes are unforgiving. Consequently, any slight error in the plant,
trucking or layout process can result in non-uniform surfaces. If the mix is gapgraded
to a suffi cient degree with a low asphalt content, it simply cannot be
produced without segregation, regardless of the techniques used.
Gap graded mixes have been successfully used in England and throughout
Europe. However, these mixes often have fi bers or polymers, enabling the use
of a higher asphalt content that makes the fi lm thicker. In many mixes a slight
increase in asphalt content (often as little as 0.2 percent) will reduce segregation
signifi cantly. Increased fi lm thickness dampens particle-to-particle contact and
reduces the tendency to separate at transfer points throughout the process.
The new SMA and Superpave mixes in the U.S. are gap-graded. However, the
addition of fi bers and polymers make them less sensitive to segregation.
A maximum density line similar to the one shown in Figure 1 can be utilized as
a guide to uniform grading. To make a chart with a maximum density line for your
operation, use a FHWA 0.45 Power Gradation Chart as shown in Figure 1. Draw
a straight line between the lower left corner of the chart and the percent point for
your largest sieve size that retains material.